southerly branch coasts in the direction of Cape Horn, and goes home to the Pacific, tired of travel; but the rest, pouring along northward, flows through the West India Islands into the Gulf of Mexico, a hollow excavated by its

stream.

It is of course to be understood that the outline of land is not caused only by the action of a current; it is determined, also, by the geological character of soil; the loose soils wear away, while rocks oppose a barrier. The West India Islands are nothing more than those hard rocky parts of an old coast line, which have withstood the constant action of a current which has been at work for ages, eating through the soft parts; so it has made a great bite in the Gulf of Mexico, and left us the West India Islands sprinkled about, as bones that proved too hard for its digestion.

In the Gulf of Mexico, encompassed by land, the water which has for a long time been acquiring warmth, offers the greatest contrast to the frosty state in which it set out on its journey. Near the mouth of the Mississippi its temperature reaches eighty-nine degrees.

If you have a thermometer which enables you to warm a little water to that point, you have only to put your finger into the warm water, and so accurately feel how far we are now from the gnawing cold of the South Pole.

As the stream flows constantly into the Gulf, it must, of course, also constantly flow out. It flows out between Florida and Cuba, being now called the Gulf Stream. This coasts northward, having a cold counter-current between it and the shore, and crosses the Atlantic south of the great bank of Newfoundland, most of it turning southward, to return by a set of counter-currents home. A branch of it, Rennel's Current, touches the Irish coast, and makes a circuit in the Bay of Biscay, sending a weak offshoot on its passage up the Irish Channel. Thus a drop of water from the South Pole, travelling by the extensive route we have just indicated, may be shaken now from the head of the

stout gentleman, who at last consents to get into his bathing machine.

We fix the water's heart in the great Southern Ocean, and it is there, not only because the intense cold of the south polar continent determines action in that direction; but because there is there also a wide expanse of sea-the widest on the globe susceptible of all impressions. The Pacific is full of natural breakwaters, reefs, shoals, and islands. At the North Pole, though there is indeed no continent, but water at the Pole itself, the lands of Europe, Asia, and America, destroy the general expanse. In the enormous reservoir of water which surrounds the lofty continent of the South Pole we find the heart of the great circulating system; and not only do the grandest ocean currents take their rise in it, but in it also commences the pulsation of the tidal wave. Household Words.

THE WINDS.

THE winds are currents of the air exactly the same in kind as those of the waters in rivers and in the seas. Every disturbance of the balance between neighbouring masses of air, an increase of density, and consequently of pressure on the one side, or a diminution of density on the other, immediately sets up a movement from the heavier towards the lighter air; just as water is set in motion if it suffers a greater pressure on one side than on the other. The most usual and general cause of such disturbances of the balance of the air is unequal heating. The causes upon which the draught of our lamps, the burning of the fuel in our fireplaces, and the airing of our rooms depend, are the same as those which we find at work as the moving power of the slightest breezes and of the mightiest gales which stir the restless air and mingle it in every climate.

The air takes its heat, in the first place, chiefly from the ground; the warmed and so expanded air rises, and it is

only in this way that the heat of the soil is spread over the higher regions of the atmosphere. Now the soil is not everywhere equally heated. The degree in which it is favored in this respect depends, as you know, greatly upon the situation and latitude of the place. Besides this, certain substances have the property of absorbing more of the sun's rays than others can, supposing equal quantities of heat to fall upon both. Again, other substances allow the sun's heat to penetrate deeper; and thus, from the one cause or the other, the soil at different places takes a different temperature. Thus, during the day-time the temperature in the shade of trees, of houses, and of clouds, in moist meadows and forests, and on surfaces of water, is usually lower than on dry soils, on rocks, on roofs, and on level plains.

The air which rises most quickly over the warmest parts of the soil, is replaced by air coming in from cooler places; and thus are set up those movements of the air which we generally find on the borders of forests, in the shade of trees, at the openings of shady mountain glens, and in such valleys themselves, on the banks of rivers and lakes, and on the sea-shore. It is impossible that the air can flow from one place to another, without being replaced by a movement in the opposite direction, for instance, by a return current in the upper regions of the atmosphere.. The following experiment will clearly illustrate this. If a door be set ajar between two rooms, one of which is filled with cold air, the other with warm, and if now a lighted candle be held at the crevice, at different heights, one after another, you will remark that at the bottom the flame will be turned from the cold room into the warm, at the top it will be driven from the warm towards the cold room, and at some point near the middle height it will burn steadily upright. From this you will learn that there are two currents, one above the other, and in opposite directions.

Just the same process goes on in the open air wherever neighbouring tracts of land have an unequal temperature

which they impart to the air that runs over them. In every hot summer's day there are streams of air mounting up from such spots on the soil as are most strongly heated; these currents carry with them the moisture as well as the warmth of the ground, and they sink again over cooler places, such as surfaces of water and forests.

This is well shown by the periodical land and sea breezes, which on many coasts blow from the sea to the land by day, and by night from land to sea. If the land is more heated than the sea during the day, the air that is over the land will mount upwards, and the cool sea air will flow into its place; the air getting cooled in the upper regions falls down again over the sea. During the night the land is more cool than the surface of the water; the latter at last becoming warmer, the air flows from the land to the sea, while the sea air, now becoming lighter, mounts upwards.

Dove* compares this circulation to the turning of a wheel. If the temperature is equal it stands still; if it become unequal it turns, first towards one side and then towards the other. Twice daily it stands still, when one of these movements is passing into the other.

Land and sea breezes occur in high latitudes only during the summer months; in tropical climates, however, they follow with the greatest regularity, and become of high importance to shipping. The sea breeze springs up in the morning some time after sunrise, increases in strength till about two or three o'clock in the afternoon, and then gradually falls off. About the time of sunset a perfect calm prevails. Soon after the land wind gets up, and blows with gradually increasing strength through the greatest part of the night, and begins to lull towards morning. These alternating air-currents appear on all coasts within the tropics, even on those of the smallest islands, and they occur every day with perfect regularity, unless they are

* Dove (Professor), of the University of Berlin,

masked or turned from their true direction by other more powerful winds. The land and sea breezes are, for the most part, felt only at a small distance from the shore. On some far-stretching coasts, however, such as that of Peru, their influence reaches a great distance out to sea.

In the hottest zone of our earth an uninterrupted current of warm air rises from over land and sea, and must be replaced from below; and thus a movement of colder air sets in from higher latitudes on both sides towards the equator. The air that has mounted up now flows in the higher regions of the atmosphere on both sides towards the poles, and falling, as it is gradually cool in its progress, reaches the earth again in the middle or higher latitudes. This is a circulation of the same kind as that presented on a far smaller scale by the land and sea breezes.

In the torrid zone, then, we find in the lower layers of the atmosphere, on both sides of the equator, polar windsblowing from the poles to the equator-north winds in the northern hemisphere and south winds in the southern, both of which, however, for a reason which I shall presently explain, take a more and more westward direction as they approach the equator. These are called the Trade-winds.

The air is in general moister over the sea than on land; least so, however, in the region of the trade-winds. It there flows from colder to warmer regions, and so, its temperature gradually increasing as it advances, its power of taking up moisture is more and more enhanced. Within the region, therefore, of the trade-winds the atmosphere is clearer and rain scarcer. But between the north-east and south-east trades, in the belt of the calms, where the surface of the sea receives most warmth, and where, therefore, air fully charged with water is ever rising and spreading itself out, and thus is cooled down by degrees below the dew-point, it rains regularly every afternoon. The domain of the calms advances, as you know, and retreats with the sun; the rainy season sets in wherever it comes; and the dry season where the trade-wind prevails. Between the